Aldehydes that evaporate from adhesives and the like contained in wallpaper and other construction materials, and ammonia and other chemical substances contained in tobacco smoke generally become causes of chemical hypersensitivity, atopy, and other maladies. Soiling of buildings and other structures due to atmospheric contamination and other factors brought about by automobile exhaust and the like damages the appearance of the buildings and causes construction materials to deteriorate.
Therefore, ceramic catalysts for adsorbing and decomposing oils and chemical substances such as aldehydes, ammonia, nitrogen oxides, and the like are being applied to metal, resin, and ceramic construction materials, and to interiors, exteriors, wallpaper, furniture, and the like. Work is also being conducted to develop metal, resin, and ceramic construction materials coated with ceramic catalysts, as well as buildings, interiors, exteriors, furniture, and other objects coated with such catalysts.
Exterior wall materials, automobiles, train cars, or other items that are used for long periods of time outdoors have particularly noticeable soiling by the atmosphere, marks from oils washed out by rain, and other types of contamination. These objects become contaminated after one or two months, they require regular washing, repainting, and other maintenance that leads to mounting costs, so materials are being sought that are maintenance-free and do not become contaminated. Marks from oily flows are particularly prominent under window frames and the like; for example, contamination is noticeable on the white tenting used in domed baseball stadiums and the like, and also on bullet trains and other train cars. This contamination is made up of carbon and other atmospheric dust that is fixed by oils, and is not easily removed by cleaning with detergents. Sick house syndrome or atopy is caused by formaldehyde, acetaldehyde, and other chemical substances emitted from construction materials or furniture. Additional examples include tobacco smoke as well as ammonia, hydrogen sulfide, methyl mercaptan, and other organic chemicals that cause pet odor from indoor dogs and the like.
Attempts have therefore been made to decompose harmful chemical substances by applying a coating containing titanium dioxide to wallpaper or interior materials, furniture, and the like. However, an organic binder cannot be used, and the coating does not function in the absence of light, so almost no effects are obtained at night or in a darkened room. There are also no examples of using such a photocatalyst inside an automobile or train car. An attempt has recently been made to use a visible light photocatalyst, but these visible light photocatalysts are also incapable of physical adsorption, so only substances that come in contact with the surface can be treated, and the effects remain inadequate even when these catalysts are applied to building materials. Furthermore, visible light-reactive photocatalysts have enhanced activity in the visible light region but extremely poor reactivity to light in the ultraviolet region.
According to its application, a coating generally takes on a yellow tinge and deteriorates over time after application due to finger marks and other oils, or due to decomposition and other processes brought about by adsorption of atmospheric moisture, oxidation, or light. Antioxidants are therefore admixed into the coating in order to enhance the durability of the coating film and to maintain its appearance. As an example of the background art, a technique has been proposed whereby a powder having photocatalyzing ability is included in a silica coating material (Japanese Laid-open Patent Application No. 8-259891). Titanium oxide, zirconium oxide, zinc oxide, strontium titanate, tin oxide, tungsten oxide, iron oxide, and bismuth oxide are cited in this publication as the types of powder having photocatalyzing ability, and examples thereof describe odor elimination, antibacterial effects, and the like as the effects obtained by including titanium oxide in a coating film.
Also proposed is a hydrophilic coating having a film that contains at least silica. The coating is formed on a film containing silica and an inorganic substance that has a mean particle diameter of 5 μm or less and is selected from the group that includes titanium dioxide, chromium oxide, and iron oxide (Japanese Laid-open Patent Application No. 5-305691). A photocatalytic hydrophilic covering film is also proposed that contains silica and a photocatalytic material selected from the group that includes TiO2, ZnO, SnO2, SrTiO3, WO3, Bi2O3, and Fe2O3 (International Publication No. WO96/29375). There is also a photocatalytic hydrophilic coating composition that is composed of crystalline titanium oxide particles, tungsten chloride, and a solution containing tetrahydrofuran (Japanese Laid-open Patent Application No. 10-237357).
Also proposed is a water-repellent material that is designed to form a hydrophobic surface and contains titanium oxide or other photocatalytic oxide particles, silicone, and a water-repellent fluororesin (Japanese Laid-open Patent Application No. 10-237431). A particulate photocatalyst in which an inactive ceramic as a photocatalyst is supported in strips on the surfaces of titania particles is also proposed as a photocatalyst particle, together with a manufacturing method thereof (Japanese Patent No. 282342). However, depending on the manner in which the catalysts are used, the odor-eliminating, antibacterial, and other effects are sometimes inadequate even when these conventional photocatalysts are included in the coating film, and there is also demand for development of a coating having high durability and ability to maintain a pleasing appearance for a long time. When an organic coating material is used as a component of a coating material, there are also drawbacks whereby the coating film deteriorates due to the organic coating material component coming into direct contact with TiO2 and the like.
Because of its strong photocatalytic activity, titanium dioxide has drawbacks in that the catalyst itself breaks down and causes discoloration, peeling, and crumbling when mixed with an organic binder and applied to paper or resin. There has conventionally been no alternative but to perform a so-called double application whereby a coating material is formed using an inorganic binder, and the coating material containing the titanium dioxide powder is applied as a top coat or the like after an undercoat has been applied, which takes two or more days to complete and is impractical, particularly in roadside building or in soundproofing walls for expressways. Such coating has only been possible on tile or other ceramic walls or metal. Making a surface hydrophilic to prevent contamination using an organic coating material and other methods have been investigated, but adequate stain-proofing effects have so far been unobtainable.
In view of such drawbacks, titanium dioxide powder coated with apatite or another calcium phosphate or a coating material has been proposed (Japanese Laid-open Patent Application No. 10-244166). Apatite is an excellent physical adsorbent with respect to protein, aldehydes, and the like, so it is capable of adsorbing substances even when there is no light present. Furthermore, this composite material is coated with apatite, so the titanium dioxide does not come in direct contact with the organic binder. As a result, the binder does not decompose even when struck by light, it becomes possible to use an organic paint, and other weaknesses of the photocatalyst are overcome. In this method, however, artificial body fluid is used; it is impossible to create apatite in the desired shape, such as a rod shape or hexagonal column; and production time or productivity is inadequate.
As a result of repeated and intense investigation conducted under these circumstances and aimed at fundamentally overcoming current photocatalyst drawbacks such as those described above, the inventors succeeded in developing a method for manufacturing a titanium dioxide photocatalyst composite material on which a compound composed of one or more apatite or other Ca9(PO4)6 calcium phosphate crystals is partially deposited by immersing titanium dioxide in an aqueous solution containing an excess of phosphorus ions and calcium ions in comparison with an artificial body fluid. As a result it became possible to manufacture a titanium dioxide photocatalyst composite material on which a compound composed of one or more units of Ca9(PO4)6 is partially deposited, in which the precipitated crystals are shaped as rods or hexagonal columns, which has excellent adsorption capacity and photoactivity (photooxidation ability), and whose specific surface area is 5 m2/g or higher.